So-called single mode optical fibre that is circularly symmetric is in fact capable of supporting two degenerate orthogonally polarised modes. Bends, twists and other irregularities are liable to introduce coupling between these modes, with the result that the polarisation state with which light is launched into one end of such fibre is soon changed as the light propagates along the fibre. In many coherent light optical fibre communications systems and sensor systems the resulting indeterminate change of state of polarisation (SoP) is unacceptable in which case use is made of fibre in which the degeneracy of the two zero-order modes is removed. The two modes are arranged to have sufficiently different propagation constants for the bends, twists and other irregularities encountered by the fibre to introduce substantially no coupling of the modes, and hence the separate identity of the two modes is preserved as light propagates down such fibre. Mode degeneracy is removed by making the fibre deliberately birefringent.
For the purposes of this specification single mode optical fibre which is designed to be sufficiently birefringent to inhibit coupling between its zero-order modes will be referred to as polarisation-preserving fibre, also known as high-birefringence fibre. Single mode fibre which has not been designed to be birefringent, and which therefore either exhibits no birefringence, or for which residual birefringence is too small to be effective in the prevention of mode coupling, will be referred to as non-polarisation-preserving single mode fibre.
Many of the systems that require the use of polarisation maintaining fibre in order to avoid the generation of indeterminate changes of SoP also require the use of branching and/or tapping elements in their optical fibre networks. Such elements are conveniently implemented in optical fibre as optical fibre directional couplers. In ordinary, non-polarisation maintaining fibre low insertion loss examples of such elements are advantageously provided by optical fibre directional couplers created by the method described in United Kingdom Patent Specification No. 2150703A (U.S. Pat. No. 4,591,372). Satisfactory results have however not been obtained when attempting to make such directional couplers of high stress-induced birefringence polarisation-preserving fibre.